This invention relates to marine seismic data acquisition and, more particularly, relates to source arrays for use in marine seismic data acquisition for hydrocarbon exploration or production purposes.
The general purpose of marine seismic data is to provide seismic images and other information related to the character of the earth formations located below the water. According to these marine seismic data acquisition techniques, source pulses are periodically generated (by a collection of individual seismic sources that make up a towed source array) at selected locations along a marine vessel's path as the vessel moves with constant speed usually along a straight line. These source array pulses travel down through the water to the water/earth interface (water or sea bottom), where they enter the earth. As these seismic pulses penetrate the earth strata under the water, some fractional amount of the energy in these pulses is reflected or refracted, and eventually some of this energy returns upwardly in the water as pressure pulses.
The marine vessel usually also tows behind it a submerged streamer cable in a straight line (often called the seismic line) being traversed by the vessel and these upwardly travelling pressure pulses may be detected by seismic detectors located in the streamer cable. Typically, these sensitive detector instruments convert the pressure pulses into electrical signals which may be routed to the vessel and recorded for further processing to derive information that may be presented as a seismic image or map of the submarine earthen area under investigation. These images or maps may then indicate the presence of any appropriate geological formations capable of holding hydrocarbons like oil and/or gas.
The source array is usually made up of a plurality of subarrays with each subarray often consisting of several individual and nonidentical seismic sources. The elements within a subarray are usually towed one behind another along a line that is near to and parallel with the seismic line, often with the larger seismic sources nearer the vessel. Typically the same marine vessel will also tow the streamer cable which contains the seismic detectors, although different vessels may be used to tow different portions of this hardware.
Various types of individual marine seismic sources are available for use in such a source array, for example, water guns, marine vibrators, and air guns. Water guns and marine vibrators tend to produce pressure pulses that have broad band frequency spectra while an individual air gun tends to produce pressure pulses that have a narrow band frequency spectra. Air guns are a very popular marine seismic source and will be the basis for further discussion, although other sources, such as water guns and marine vibrators, may also be used as marine seismic sources.
An air gun is towed at some depth (from a few feet to a few tens of feet) below the surface of the water and, in general, suddenly releases a confined volume of high pressure air, thus radiating seismic energy in all directions. Typically, the near-field waveform generated by a single air gun is a long series of damped oscillatory pressure pulses resulting in a corresponding periodic frequency spectrum having a fundamental frequency and multiples thereof that are related to the depth of the gun, and the gun's operating pressure and chamber volume. In addition to this undesirable long pulse, a single air gun is a relatively weak energy source, so that any electrical signal derived from subsurface reflections or refractions will likely have a low value. These features make a single air gun undesirable for marine seismic data acquisition. To improve both the signal strength and the signal shape it has been proposed and is well known to use a plurality of air gun sources belonging to a so-called "tuned array".
It has been found advantageous in marine seismic data acquisition to use a plurality of individual air guns within such an array in order to provide a composite vertically downgoing source array pulse of satisfactory amplitude and sufficient frequency content. More particularly, it has been found desirable to simultaneously generate a number of pressure fields from air guns having various fundamental frequencies and multiples thereof to provide a composite vertically downgoing array pulse having high energy and a broad frequency spectrum. That is, air guns at the same depth and supplied by the same high pressure air source, but having various chamber volume capacities are generally used in such arrays in order to produce an energetic (high amplitude) vertically downgoing composite array pulse having a broad frequency band.
The air gun with largest chamber volume provides the lowest fundamental frequency. The other guns have fundamental frequencies that fill in between this lowest frequency and twice this lowest fundamental frequency (first harmonic). In this way the frequency spectrum or band is essentially completely filled in since these other guns have frequency spectra that also repeat at multiples of their fundamental frequencies. In general, these fundamental frequencies also depend on interactions among the pressure fields radiated by the air guns, i.e., on the array geometry. Hence, air gun array design is complicated due to the interactions among the individual elements.
Array design is simplified when array elements radiate the same waveform regardless of their location in the array. For example, marine vibrators and waterguns more accurately approximate an element (for typical spacing) whose outgoing waveform is independent of position in the array than an air gun. Air gun interaction does, however, tend to broaden the fundamental frequency and its harmonics thereby helping to fill in the array pulse's spectrum. It is important to note that each particular chamber volume is roughly responsible for a different portion of the composite pulse's frequency spectrum. Many such air gun arrays have been designed and implemented using an increasing number of guns and an increasing total volume of compressed air.
In addition, it is known to construct an array from several subarrays, with each subarray being a separate collection of air guns of different chamber volumes designed to provide a desirable vertically downgoing pulse. There may be several of these subarrays that are towed in parallel behind the vessel, i.e. a "cross- line" fashion, to provide a so-called "super wide array". Alternatively, many of these subarrays may be towed in a serial manner behind the vessel, i.e. an "in-line" fashion, to provide a so-called "super long array".
In general, such prior array designs usually attempt to solve the problem of maximizing the amplitude spectrum over a desired frequency range in order to ensure that there is a satisfactory vertically downgoing, broad frequency band, composite array pulse having desired peak-to-peak and peak-to-bubble values. Although it is known that an array pulse's amplitude and phase spectra depend upon the angle of emergence from the array, this effect is usually ignored.
Thus, the amplitude and phase spectra of a far-field generally downgoing pulse for an array can be related to the deviation from the downgoing vertical direction, as measured by the angle of emergence from the array (or emergence angle), or equivalently the angle of incidence into the earth. In the subsequent processing of the recorded seismic data it is sometimes desirable to remove any array pulse's far-field phase spectrum from the data. This far-field phase spectrum depends upon the array geometry and notional waveforms radiated by the individual sources. With this information (array geometry and notional waveforms), it is possible to correct for the changes in phase as a function of emergence or incidence angle. This is a fairly large amount of signature information and each incidence angle receives its own unique correction. This level of effort is usually not put forth. Accordingly, it is currently conventional practice to ignore any phase variation with incidence angle and to assume that the phase spectra of source array pulses radiated into incidence angles of interest are identical with that of the vertically downgoing pulse even though this assumption is not justified for typical source array designs.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for a marine seismic source array having controlled phase.